This invention relates to methods of and apparatus for forming hollow metal articles utilizing internal fluid pressure to expand a hollow metal preform or workpiece against a die cavity, and especially to pressure-ram-forming methods and apparatus and the like. In an important specific sense, the invention is directed to methods of and apparatus for forming aluminum or other hollow metal articles having a contoured shape, e.g. such as a bottle shape with asymmetrical features. For purposes of illustration particular reference will be made herein to forming metal containers, but the invention in its broader aspects is not limited thereto.
Metal cans are well known and widely used for beverages. Present day beverage can bodies, whether one-piece “drawn and ironed” bodies, or bodies open at both ends (with a separate closure member at the bottom as well as at the top), generally have simple upright cylindrical side walls. It is sometimes desired, for reasons of aesthetics, consumer appeal and/or product identification, to impart a different and more complex shape to the side wall and/or bottom of a metal beverage container, and in particular, to provide a metal container with the shape of a bottle rather than an ordinary cylindrical can shape. Conventional can-producing operations, however, do not achieve such configurations.
Copending U.S. patent application Ser. No. 10/284,912 (patent application Publication No. US 2003/0084694 A1), now allowed, the entire disclosure of which is incorporated herein by this reference, describes convenient and effective methods of and apparatus for forming metal workpieces into hollow metal articles having bottle shapes or other complex shapes, including methods and apparatus capable of forming contoured shapes that are not radially symmetrical, to enhance the variety of designs obtainable.
In particular, copending application Ser. No. 10/284,912 describes a method of forming a hollow metal article such as a container of defined shape and lateral dimensions, comprising disposing a hollow metal preform having a closed end in a die cavity laterally enclosed by a die wall defining the shape and lateral dimensions, with a punch located at one end of the cavity and translatable into the cavity, the preform closed end being positioned in proximate facing relation to the punch and at least a portion of the preform being initially spaced inwardly from the die wall; subjecting the preform to net internal fluid pressure to expand the preform outwardly into substantially full contact with the die wall, thereby to impart the defined shape and lateral dimensions to the preform, the fluid pressure exerting force, on the preform closed end, directed toward the aforesaid one end of the cavity; and, either before or after the preform begins to expand but before expansion of the preform is complete, translating the punch into the cavity to engage and displace the closed end of the preform in a direction opposite to the direction of force exerted by fluid pressure thereon, deforming the closed end of the preform. Translation of the punch is effected by a ram which is capable of applying sufficient force to the punch to displace and deform the preform. This method is referred to as a pressure-ram-forming (PRF) procedure, because the container is formed both by applied internal fluid pressure and by the translation of the punch by the ram. The term “net internal fluid pressure” as used herein means a positive interior-to-exterior pressure differential across the preform wall.
The punch has a contoured (e.g. domed) surface, the closed end of the preform being deformed so as to conform to the contoured surface. The die cavity has a long axis, with the preform having a long axis and being disposed substantially coaxially within the cavity, and the punch being translatable along the long axis of the cavity. When the die wall comprises a split die (made up of two or more mating segments around the periphery of the die cavity) separable for removal of the formed hollow metal articles, the defined shape may be asymmetric about the long axis of the cavity; i.e., PRF forming can produce an asymmetric profile (for example, feet on the bottom or spiral ribs on the side of the container).
The punch is preferably initially positioned close to or in contact with the preform closed end, before the application of fluid pressure, in order to limit axial lengthening of the preform by the fluid pressure. Translation of the punch may be initiated after the expanding lower portion of the preform has come into contact with the die wall.
The preform, especially when the hollow metal article to be formed is a bottle-shaped container or the like, is preferably an elongated and initially generally cylindrical workpiece having an open end opposite its closed end. It may be substantially equal in diameter to the neck portion of the bottle shape, and may have sufficient formability to be expandable to the defined shape in a single pressure forming operation. If it lacks such formability, preliminary steps of placing the workpiece in a die cavity smaller than the first-mentioned die cavity, and subjecting the workpiece therein to internal fluid pressure to expand the workpiece to an intermediate size and shape smaller than the defined shape and lateral dimensions, are performed prior to the PRF method described above. Alternatively, if the elongated and initially generally cylindrical workpiece is larger in initial diameter than the neck portion of the bottle shape, the method of forming a bottle-shaped container may include a step of subjecting the workpiece, adjacent its open end, to a necking operation to form a neck portion of reduced diameter, after performance of the PRF procedure; or the diameter of the neck area of the preform can be reduced using a die necking procedure which may be applied before the expansion stage.
During the step of subjecting the preform to internal fluid pressure, the fluid pressure within the preform occurs in successive stages of (i) rising to a first peak before expansion of the preform begins, (ii) dropping to a minimum value as expansion commences, (iii) rising gradually to an intermediate value as expansion proceeds until the preform is in extended though not complete contact with the die wall, and (iv) rising from the intermediate pressure during completion of preform expansion. Stated with reference to this sequence of pressure stages, the initiation of translation of the punch to displace and deform the closed end of the preform in a preferred embodiment of the invention occurs substantially at the end of stage (iii).
Typically, when the internal fluid pressure is applied, the closed end of the preform assumes an enlarged and generally hemispherical configuration as the preform comes into contact with the die wall; and initiation of translation of the punch occurs substantially at the time that the preform closed end assumes this configuration.
The step of subjecting the preform to internal fluid pressure may comprise simultaneously applying internal positive fluid pressure and external positive fluid pressure to the preform in the cavity, the internal positive fluid pressure being higher than the external positive fluid pressure. The internal and external pressure are respectively provided by two independently controllable pressure systems. Strain rate in the preform is controlled by independently controlling the internal and external positive fluid pressures to which the preform is simultaneously subjected for varying the differential between the internal positive fluid pressure and the external positive fluid pressure. In this way, more precise control of the strain rates may be achieved. In addition, the increased hydrostatic pressure may reduce deleterious effects of damage (voids) associated with the microstructure of the material.
Heat may be applied during expansion of the preform, so as to induce a temperature gradient in the preform. By adding heaters to the punch, a temperature gradient is induced in the preform from the bottom up. Separate heaters may be added at the top of the die which induce a temperature gradient in the preform from the top down. Further heaters may be included in the side walls of the die cavity.
It has also been found advantageous to have the punch in contact with the bottom of the preform before the start of the expansion phase and to apply some axial load by the punch throughout the expansion phase. With this procedure where the punch applies some axial load to the closed end of the preform throughout the expansion phase, the displacement and deformation of the preform closed end are preferably not carried out until completion of the expansion phase.
Internal and external positive fluid pressures may be applied by feeding gas to the interior of the preform and to the die cavity externally of the preform, respectively, through separate channels. Heat may be applied to the preform by multiple groups of heating elements respectively incorporated in upper and lower portions of the die structure and under independent temperature control for controlling temperature gradient in the preform. Additionally or alternatively, heat may be applied to the preform by a heating element disposed within the preform substantially coaxially therewith; and heat may be further supplied to the preform by heating the punch.
In addition, where the neck portion of the defined container shape includes a screw thread or lug for securing a screw closure to the formed article, and/or a neck ring, the die wall may have a neck portion with a thread or lug formed therein for imparting a thread to the preform during expansion of the preform.
Heretofore, in pressure-ram-forming operations emphasis has been given to the reliable production of articles such as containers to meet customer requirements, utilizing pressures which are “safe” (from the standpoint of avoiding failures) and consequent relatively long cycle times. As used herein, “failure” means a structural flaw such as a pinhole or split in the produced article, resulting from a defect in the manufacture of the preform and/or an inherent limit to the formability of the alloy.
For the sake of manufacturing economy, however, it would be desirable to decrease the cycle time (time for forming one container or other article) of the PRF process while achieving acceptable forming properties and, in particular, avoiding failures in the produced articles. More generally, it would be desirable to achieve improved computer control of complex forming processes such as the PRF process.